The purpose of this research is to contribute to our understanding of the neuronal machinery responsible for learning and response selection in complex, integrated behavioral repertoires. These related problems are to be studied in a relatively simple system, the crayfish escape-response repertoire, which consists of a small set of escape responses all of which are subject to habituation and probably also to other forms of plastic modification. Specific behavioral and electrophysiological research is proposed on (1) cellular mechanisms of synaptic depression that is responsible for short-term habituation, (2) causes of long-term habituation, (3) the role of the neuron genome in long-term plasticity, (4) integration of escape behavior, and (5) a variety of forms of plasticity other than habituation.